Photo Albums

February 2008

westone

January 10, 2008

What is slate

is considered the lowest grade of metamorphic rock. It forms when shale, which consists of clay minerals like the sediments from which it formed, is put under pressure with temperatures of a few hundred degrees or so. Then the clays begin to revert to mica minerals. This does two things: first, the surface of slate has a sheen and reflectance typical of mica, and second, the rock gets a pronounced cleavage direction, so that it breaks along flat planes. This new cleavage plane is not always the same as the original sedimentary bedding planes, and any fossils originally in the rock are usually erased. With further metamorphism, slate turns to phyllite, then schist or gneiss.

Slate is usually dark, but it can be colorful too. High-quality slate is an excellent paving stone as well as a raw material for long-lasting roof tiles. Blackboards and handheld writing tablets were once made of slate, and the name of the rock has become the name of the tablets themselves.

This slate paving stone has two interesting features. On the lower left side you can see the point where the quarry worker struck the stone free from its block—the fracture surfaces radiate from that point. And along the top are some crystals of pyrite that grew in the rock as it metamorphosed. See a closeup on the pyrite page in the Mineral Gallery.